1. Field of the Invention
This invention relates to a process for producing a styrenic resin composition, more particularly to a process for preparing a styrenic resin composition that contains a styrenic resin and an organic phosphorus-containing antioxidant.
2. Description of the Related Art
Styrenic resins, such as acrylonitrile-butadiene styrene copolymeric resin (ABS resin), can be used for the production of automobile parts, electronic parts and office automation appliances, due to their high impact strength, good stiffness, and excellent characteristic for molding and processing.
ABS resin is usually prepared by compounding styrene-acrylonitrile copolymer prepared by solution polymerization techniques and dry powdery acrylonitrile-butadiene-styrene grafted copolymer prepared by emulsion polymerization techniques with lubricants and stabilizers, such as phosphorous-containing antioxidant (e.g. Tris nonyl phenyl phosphite) via in an extruder at a temperature of 180xc2x0 C. or higher. Usually, preparation of the dry powdery acrylonitrile-butadiene-styrene graft copolymer having a water content of less than 1 wt % involves the steps of coagulation and dewatering of the ABS graft copolymer latex to get the wet powdery acrylonitrile-butadiene-styrene grafted copolymer having a water content ranging from 15 to 40 wt % and followed by drying of the wet powdery graft copolymer with hot air. The process is complicated and uneconomical. Moreover, there is also a high risk of fire during the drying of the wet powder. The thus-formed acrylonitrile-butadiene-styrene graft copolymer powder having been dried with hot air tends to result in yellowish in appearance and poor thermal stability.
Japanese Publication No. 59-37021 discloses a method for producing ABS resin by extruding the aforesaid coagulated and dewatered wet powdery acrylonitrile-butadiene-styrene graft copolymer and styrene-acrylonitrile copolymer in an extruder that includes a silt type dewatering section and vent ports. Though the method dispenses with the aforesaid drying step, high extrusion temperature (above 200xc2x0 C.) and high water content of the wet powder results in high moisture content in the extruder, which, in turn, results in hydrolysis of the phosphorous-containing antioxidant blended therewith, thereby resulting in poor thermal stability of the ABS resin. Furthermore, tris nonyl phenyl phosphite is one of the commonly used phosphorous-containing antioxidants for ABS resin, and it can be hydrolyzed to form nonyl phenol which has been shown to have an effect to interfere endocrine of human body and animal body and thus is harmful to the environment. Therefore, for a long time of the ABS industry, it is necessary to overcome the drawback of poor stability to hydrolysis of the phosphorous-containing antioxidants during processing of ABS resin, especially at a high temperature and a high humidity condition during extrusion as encountered in the aforesaid method.
Phosphorous-containing antioxidants tend to hydrolyze very easily at a high temperature and high moisture condition. Such hydrolysis generates phosphoric acid, thereby further expediting the rate of the hydrolysis. As such, the hydrolysis not only decreases the antioxidative property of the composition, but also causes damages to equipment, and brings about undesirable pollution. Accordingly, there is a long need in this industry to avoid hydrolysis of phosphorous containing antioxidant during the production of ABS resin. In order to solve the disadvantages described above, it is normally proposed to add organic amines in the reaction to decrease the rate of the hydrolysis of the phosphorus-containing antioxidant. Examples of the amines described above are hexamethylene tetramine, triisopropanoyl amine, stearyl dimethyl amine, and the like. However, the improvement is not satisfied, and the presence of the amine will affect the chemical property of the resin and generate poison as well. Therefore, the amine is inappropriate for use. Moreover, organic amines are normally used along with liquid phosphide antioxidant, such as tri-nonyl-phenyl phosphite, di-phenyl isodecyl phosphite, di-phenyl-isooctyl phosphite, tri-phenyl phosphite, 4,4xe2x80x2-iso-propylidene-diphenol-alkyl (C12-15) phosphite, and the like.
Another method for stabilizing against hydrolysis is to enhance the sterical hindrance of the substitutent (such as phenolic group) at the ortho-position phenolic to hinder water from contacting the phosphorus, thereby effectively preventing the occurrence of hydrolysis. One such example is an antioxidant Irgafos 168 (a trade name, available from Ciba geigy Chemicals Corporation) which is tris(di-tert-butyl-2,4-phenyl) phosphite and which consists of three phenolic groups substituted at the ortho-position with tertiary butyl substitutents and which has the following formula (I): 
While hydrolysis of the antioxidant of formula (I) can be reduced, however, the composition also reduces the affinity of the phosphorus for oxygen, thereby weakening the antioxidative property. As such, more phosphorus containing antioxidant has to be added in order to achieve the same property.
To improve the disadvantage described above, U.S. Pat. No. 5,856,550 proposed another method for stabilizing against hydrolysis by adding inorganic acid-binding agent into the phosphorus-containing antioxidant in combination with the amines to achieve the purpose for stabilizing against hydrolysis. Examples of the inorganic acid-binding agent are metal oxide, metal carbonate, metal carboxylate, hydrotalcite, zeolite, and the like. By using the inorganic acid-binding agent in combination with triisopropanyl amine to absorb the phosphoric acid generated from the hydrolysis of the phosphorus-containing antioxidant, the rate of the hydrolysis in an acid condition can be deterred. However, since the affinity of the inorganic acid binding agent for the organic phosphorus compounds is weak, poor stabilization of the phosphorus-containing antioxidant results when the agent is non-uniformly distributed. Moreover, the addition of organic amines will result in problems, such as initial color of the resin, thermal stability during processing, and generation of toxic gas.
U.S. Pat. Nos. 4,810,579 and 5,326,803 disclose the use of reactive coupling agent for encapsulating the surface of the organic phosphorus compounds, thereby utilizing the hydrophobic characteristic of the coupling agent for water to achieve the purpose of stabilizing against hydrolysis. While the methods described above may be useful for powdered organic phosphorus compounds stored in a high atmospheric humidity, they are still insufficient to effectively stabilize against hydrolysis when they are subjected to a high humidity condition with a temperature above 200xc2x0 C. For example, in case of blending the organic phosphorus compounds with the resin, followed by extruding from an extruder at a high humidity and a high temperature (above 100xc2x0 C.), the stability to hydrolysis cannot be achieved with the above described methods.
U.S. Pat. No. 4,820,772 discloses a composition obtained by blending high density ethylene polymer and non-crystalline ethylene-propylene random polymer. Additives, such as organic phosphite compounds and zeolite can be added into the composition. The zeolite used can be silane treated.
U.S. Pat. No. 4,923,918 discloses a method for producing a modified propylene polymer. The method involves blending a mixture of propylene polymer, phenolic antioxidant, radical generator, and zinc salt of a carboxylic acid. Additives, such as recite zeolite (which can be silane treated) and organic phosphites, can be added into the propylene polymer.
U.S. Pat. No. 5,001,176 discloses a crystalline polyolefin composition that contains crystalline polyolefin, dibenzylidenesorbital type compound, and cyclodextrin. Additives, such as zeolites, silane coupling agents and organic phosphites, can be added into the polyolefin.
U.S. Pat. No. 5,141,995 discloses a modified propylene polymer prepared by blending propylene polymer with polyol or partial ester of polyol, phenolic antioxidant, and radical generating agent. Additives, such as recite zeolite, and organic phosphites can be added into the polymer.
U.S. Pat. No. 6,005,034 discloses a method for producing propylene-ethylene copolymer. Additives, such as zeolite, silane coupling agent and organic phosphites, can be added into dried propylene-ethylene copolymer.
Unlike the aforesaid wet powdery acrylonitrile-butadiene-styrene graft copolymer for the preparation of ABS resin, the raw materials (ethylene polymer, propylene polymer, ethylene-propylene copolymer) used in the aforementioned patents are free of water and have no adverse effect on the organic phosphites. Therefore, these patents fail to teach how the aforesaid hydrolysis problem can be solved via the use of zeolite treated with silane and organic phosphites during extrusion at a high humidity and a high temperature condition. Moreover, organic phosphites, zeolite and the polymers employed in these patents are added into a mixer or an extruder, respectively. Therefore, none of these patents teaches mixing of organic phosphites and zeolite prior to mixing or compounding with the polymers in order to enhance stability of organic phosphites against hydrolysis.
U.S. Pat. No. 4,138,363 discloses a hydrophilic silane-zeolite composition and a method for producing the same. The composition is formed by condensing a hydrophilic silane onto the surface of a hydrated zeolite. The patent fails to teach neither the application of the composition to styrenic resin nor the stability of hydrolysis of phosphorus-containing anti-oxidant processed at a high temperature and a high humidity condition.
Therefore, it is proposed to overcome the problems described above by modifying the composition of the organic phosphorus containing antioxidant so as to render the same to exhibit good stability to hydrolysis when it is subjected to a high temperature and a high humidity condition.
Therefore, it is an object of this invention to provide an organic phosphorus-containing antioxidant which exhibits good stability to hydrolysis, and which results in good initial color of the resin composition, and which enhances thermal stability during processing and eliminates the generation of toxic gas.
It is another object of this invention to provide an organic phosphorus-containing antioxidant which is capable of enhancing the processability and stability to hydrolysis of resins which are blended with the former, and which can further comprise a carrier and then added directly to the resins.
Thus, this invention is characterized by an organic phosphorus-containing antioxidant comprising: an organic phosphite or organic phosphonite; and an acid-binding metal salt treated with a surface conditioning agent. A carrier can be further included as needed to form an antioxidant which exhibits excellent stability to hydrolysis and antioxidative property when it is subjected to a high temperature and a high atmospheric humidity condition. The above described antioxidant can be used as additives for polymers.
It is a further object of this invention to provide a process for producing a styrenic resin composition comprising the steps of compounding 100 parts by weight of a styrenic resin having a water content greater than 2% by weight, and 0.01 5.0 parts by weight of an organic phosphorus-containing antioxidant in an extruder to form the styrenic resin composition. The phosphorous-containing antioxidant is formed by mixing (a) 20 to 99.9 wt % of organic phosphite or phosphonite with (b) 0.1 to 80 wt % of acid-binding metal salt treated with a surface conditioning agent, prior to mixing or compounding with the styrenic resin. The weight ratio of the surface conditioning agent to the acid-binding metal salt is 0.1 to 50 wt %:99.9 to 50 wt %. The surface conditioning agent is selected from the group consisting of silane coupling agents, titanate coupling agents, zirconate coupling agents, aluminate coupling agents, zirco-aluminate coupling agents, non-ionic surfactants, and anionic surfactants. The styrenic resin having a water content greater than 2% by weight comprises 5-100 wt % of a rubbery graft copolymer (A) and 95-0 wt % of a styrenic copolymer(B), based on the total weight of the styrenic resin (dry base). The rubbery graft copolymer(A) is prepared by graft polymerization of 50-90 wt % of styrenic monomer, 10-50 wt % of vinyl cyanide monomer, and 0-10 wt % of copolymerizable monomer in the presence of a rubbery polymer latex, followed by coagulation and recovery to have a water content of 5-50 wt %. The styrenic copolymer(B) is prepared by coploymerization of 50-90 wt % of styrenic monomer, 10-50 wt % of vinyl cyanide monomer, and 0 40 wt % of copolymerizable monomer.